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Wet Season and Dry Season The Onset of the Wet and Dry Seasons in East Central Florida- A Subtropical Wet-Dry Climate? by Randy Lascody March 2002 1. Introduction East central Florida experiences seasons that differ from most of the remainder of the country. Rather than the four seasons of winter, spring, summer and fall, east central Florida exhibits a distinct Wet (warm) Season and Dry (cooler) Season. This duality of seasons is similar to the Monsoon or Wet-Dry climates that other regions of the world experience. While the general onset of the Wet and Dry Seasons is known by most, a quantitative determination has not been previously accomplished for east central Florida. The Wet Season The Wet Season is typically considered to begin in the latter part of May and resembles "summer" across much of the remainder of the country. Warm temperatures (mid to upper 80s) begin earlier, but usually do not coincide with the beginning of frequent summer-like rains. The primary difference between summers in east central Florida and those at higher latitudes is that the heat and humidity are relentless (i.e. there are no synoptic scale fronts that bring significant cooling and drying). Though it does not rain every day during the summer, the frequency of rainfall usually begins to increase in late May. The start of the Wet Season is occasionally delayed until June and in rare cases, even as late as early July (e.g. 1998). The Dry Season The Dry Season usually begins in October as the first synoptic scale cold front brings drier and slightly cooler air into the area. This first front sometimes results in a significant rain event. Tropical systems, additional fronts and gale centers can bring periods of heavy rain through November, but the frequency of rain almost always decreases after the first significant frontal passage. Fronts continue to push through the area during the traditional "meteorological winter" months of December, January and February. Cold frontal passages during this time period will sometimes be preceded by a line of showers and thunderstorms, but the occurrence of rainfall is much less frequent than the summer. The greatest coverage of rainfall during the winter months often occurs when one of these fronts moves back northward as a warm front. Some winters have more frequent frontal passages, which can result in much above normal rainfall. The most recent occurrence was the 1997-1998 winter when a strong El Nino resulted in significant severe weather episodes and flooding across the Florida peninsula. For east central Florida, late February through March might be the time period that most closely resembles typical Spring weather in the higher latitudes. Large swings in temperatures often occur along with occasional severe weather episodes, but rainfall is usually infrequent. April is often the driest month of the year as fronts become weaker and yield less rainfall, yet manage to pass through the area and reinforce the dry and stable air mass. Temperatures warm through May with average maximum readings reaching the upper 80s by the end of the month. Rainfall frequency increases compared to April, with the most notable increase usually beginning late in the month. 2. Methodology Biedinger and Lushine1 conducted a study to determine the onset/end of the Rainy Season in southeast Florida. Their methodology was to determine when the minimum temperature and surface dew point first began to equal or exceed 70 degrees. This constituted the beginning of the Wet Season. The end of the Wet Season was then logically determined to be when minimum temperatures and surface dew points began to fall below 70 degrees. The frequency of rainfall almost always increased (decreased) as dew points remained above (below) 70 degrees. This method was deemed inappropriate for determining the onset of the Wet Season in east central Florida since the frequency of rainfall often increases before dew points and minimum temperatures stay above 70 degrees. This is most likely due to the lower sea surface temperatures adjacent to east central Florida. The Gulf Stream being farther offshore compared to south Florida is the primary reason for the delay in the near shore sea surface temperature rise. Another minor factor for the delay in heating of the waters is a slightly lower sun angle compared to south Florida. Heating of land areas in east central Florida is also affected by the sun angle. In some years this may be a small factor that delays the destabilization of the atmosphere and the associated onset of more frequent showers and thunderstorms along the Gulf of Mexico and Atlantic sea breeze boundaries. The onset of the Dry Season was also not always associated with the first fall in dew points/minimum temperatures below 70 degrees. In fact, average temperatures start to fall below 70 degrees in early October. A better measure of the onset of the Dry Season correlates to when a front drops temperatures below 60 degrees for the first time. Two different methodologies were used in this study to determine the onset of the Wet and Dry Seasons. The first method involved a laborious inspection of Local Climatological Data (LCD) from Orlando and Daytona Beach. Data from the Orlando Herndon Airport (now Orlando Executive Airport) was examined from 1949 to 1974 while Orlando International Airport data was used during the 1975- 2000 period. This data was inspected from the National Climate Data Center's (NCDC) website. A local archive of data from Daytona Beach was available for the 1935-2001 period. The second method used online meteorological data from the University Corporation for Atmospheric Research (UCAR). The data utilized in this method was the rainfall rate and precipitable water averaged over a one degree by one degree area between 28-29N latitude and 81-82W longitude. Method 1 -- Wet Season For Orlando, data for the period 1949-2001 was examined. Minimum temperatures in the 67-70 degree range were found to coincide with the onset of the "humid" season. This usually occurred a few days before the start of more frequent showers/thunderstorms. The median date for the onset of the Wet Season at the Orlando Airports (International Airport and the old Herndon Airport) was found to be May 24 (Table 1). For Daytona Beach, the 1935-2001 period of record showed that there were more frequent intrusions of drier air after the apparent beginning of the humid/wet season. Nevertheless, the median date for the start of the Wet Season in Daytona Beach was determined to be May 27 (Table 1), which is similar to Orlando. It must be stated that a purely objective analysis is not possible since the exact onset of the Wet Season is difficult to determine in some years. There were classic years when dew points and minimum temperatures rose to around 70 in mid/late May, a rainy period ensued shortly thereafter and continued through the summer. Some years saw the start of showers/storms in late May, followed by several weeks of little/no rain, and then the onset of frequent rains once again in late June. Attempts to objectively pick the date when dew points/minimum temperatures remained above 70 degrees failed, since many years had brief periods of readings in the 60s through the month of June. This would have resulted in the median date for the onset of the Wet Season not correlating with a reasonable person's perception, and being much too late in the season. Method 1 -- Dry (Cool) Season Orlando: The median date for the onset of the Dry Season was found to be October 15. Daytona Beach: The expanded period of record from 1935-2001 yielded the same median date for the onset of the Dry Season, October 15. Table 2 summarizes the subjectively determined dates of the Dry Season onset for each year. Exact dates for the onset of the Dry Season are difficult to determine in some years. Average minimum temperatures remain in the 70s throughout the wet (warm) season, and start to slip into the upper 60s during early October. There are classic years where a front lowers dew points and minimum temperatures below 60 degrees during the first or second week of October and rainfall frequency sharply diminishes. However, some years may see a cold frontal passage during late September or early October, followed by a period with little/no rain, then see rainfall frequency increase through the rest of October. A strong cold front will usually dry things out significantly in early November. For this type of scenario, one person analyzing the data might pick late September as the onset of the Dry Season, but another might choose early November. Rainfall frequency in a few years actually decreased before humidities lowered. Therefore it was concluded that a purely objective determination of the Dry Season onset is not possible. A subjective analysis was necessary for many years. To bring more objectivity to this study, the date of the first minimum temperature below 60 degrees was determined for the late September to early November period. In many years, this was the air mass that put an end to the Wet Season. However, sometimes this cool/dry air mass would occur very early (i.e. September 20, 1981), then temperatures would warm back up and the air mass would moisten, leading to a period of summer like weather with more frequent rainfall. This also occurred in 2001 when a frontal passage on September 30 apparently put an end to the Wet Season, but very warm temperatures returned after about 5 days. Rainfall returned over portions of east central Florida between two more cold fronts that passed through during October.
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